It is possible to draw an image such as a character, a mark or the like on an object such as a thermal rewritable medium or the like by irradiating a laser light (by which the temperature of the irradiated part of the object becomes high by absorbing the laser light) and heat. A laser drawing apparatus (a laser irradiating apparatus or a laser marker) using this technique has been developed and commercially available (patent document 1 to patent document 5, for example).
The laser drawing apparatus uses a gas laser, a solid laser, a liquid laser, a semiconductor laser or the like for a laser light source and an image such as a character, a mark or the like can be drawn on the object. The object may be a medium to which writing and deleting can be performed by changing the applied temperature. The object may be capable of being repeatedly used, such as a metal, a plastic, a thermal paper, a thermal rewritable medium or the like by the oscillatory wavelength of the laser light.
For the metal or the plastic, the image can be drawn by ablating or baking by irradiating the laser light to heat the surface of the object. For the thermal paper or the thermal rewritable medium, the image can be drawn by developing color of a recording layer by irradiating the laser light to heat the recording layer of the object.
FIG. 1 is an explanatory view for explaining drawing of a single stroke drawing by irradiating a laser light. Here, the single stroke drawing is a line segment.
In FIG. 1, the center of the laser light is moved (or scanned) from the first end point P1 to the second end point P2 to draw a line segment. The circle shown around the center of the laser light is an outline of the beam of the laser light. For this case, as the part of the object where the beam of the laser light is irradiated is heated, the length of the drawn line segment is expected to be L from the outer edge (left side edge) of the beam at the first end point P1 to the outer edge (right side edge) at the second end point P2.
As shown by the dotted lines in FIG. 1, the beam of the laser light is irradiated to continuously overlap as the center of the laser light is moved (or scanned) at the part other than the edges of the line segment. Therefore, although the heat diffuses toward the direction where the beam of the laser light is not irradiated, the object is heated enough to be ablated or baked when the object is the metal or the plastic, or to develop color when the object is the thermal paper or the thermal rewritable medium at the part where the beam of the laser light is irradiated to overlap.
However, the temperature at the edges of the line segment, where the beam of the laser light is not irradiated to overlap, may not become high enough to be ablated or baked when the object is the metal or the plastic, or to develop color when the object is the thermal paper or the thermal rewritable medium. As a result, the line segment is not drawn at the edges which are intended to be drawn so that the length of the line segment becomes L′ which is shorter than the intended length L, reducing drawing quality.
It is described in Patent document 5 that the edges of a line segment are extended by extending the length of irradiating a laser light for a certain fixed length to reduce the influence of the above problem. However, it is difficult to appropriately extend the length of the line segment by extending the length of irradiating the laser light for a fixed length.
The present inventors have found that the shortened length of the line segment may differ based on characteristics of the object or the scanning speed of the laser light.